This is a small infrared source to quickly check the health status of photodiode amplifiers. It is equipped with an af source generating pulses of approx. 999 Hz which are then used to do some ask on a 32.768 kHz (1.00 MHz) carrier, delivered by an xco. And yes, we used this calculator :-)

The device is battery powered by 2 x AAA, 3 V. (Battery Case: Keystone 2468)

And yes, you could also use an infrared remote control. Make sure you press a button which sends a constant datastream, e.g. Volume up/down.

The diode used is a 15400585F3590 from Würth Electronics (Mouser, CHF 7.87 per 10). It has the following Properties :

Properties			min	typ	max	Unit
Peak Wavelength	λpeak	50 mA		850		nm
Centroid Wavelength	λcentroid	50 mA		845		nm
Radiant Intensity	Ie	50 mA	35	85		mW/sr
Forward Voltage	Vf	50 mA		1.5	2.2	V
Spectral Bandwidth	Δλ	50 mA		40		nm
Viewing Angle	2θ50%	50 mA		30		°

The Spectrum, as shown in the datasheet of Würth Electronics

The upper trace in yellow shows the output of the LMC555 timer. The frequency determining components are R1 = 5k6, R2 = 4k3 and C1 = 100 nF, which shall produce a 1.014 kHz squarewave. The middle trace (violet) shows the output of the crystal oscillator. 32.768 kHz is assembled here. Those two signals are logically AND-ed to result in the blue (bottom) trace, which is used to switch the IR diode on/off.

The first experiment is done by using a Receiver made with the same IR LED, soldered to a BNC connector and plugged into a high impedance input of an oscilloscope. Depending on the distance, we can see our modulated as well as demodulated signal.

Now for the second experiment, we added a 50 Ω termination between the diode and the scope. We see, that the signal decreases much in amplitude. We therefore compare the IR LED to a Si Photodiode (BPX-65), which is optimised for 'RX' (from 350 nm to 1100 nm). Polarity not important.

We see, that the Si Photodiode produces a much cleaner signal, higher in amplitude. We also see, that a BPX-65 can drive a high current into a 50 Ω load (when very close) to produce a voltage of approx. 10 mVpp (which corresponds to ≈ 0.2 mApp). This will decrease with distance and power used by the transmitter.

That's where the amplifier becomes part of the game. Comparing some Photodiode - Amplifiers, we see, that the gain is usually high (10 ... 1000). That means that approaching a Photodiode - Amplifier with our Source must result in some sort of Saturation - or a low amplitude when an amplifier stage is broken.

The QO-EL-0010 is a Photodiode Amplifier with a BPX-65 and 2 x GALI-39+. This would result in a gain of approx. 40 dB (@ 0.1 - 1 GHz) ≈ Vu = 100. Distance is ≈ 10 cm. Minus the 40 dB introduced by the TCBT-14+ at 30 kHz. Therefore, we would expect a signal of U ≈ 10 mVpp

We can see, that functional Photodiodes produce an output which is clean and high in amplitude.

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